Wear and corrosion resistant coatings containing precious metals, in particular gold, have already been described many times. What is sought is a product having good scratch resistance while presenting the aspect and brilliance of gold.
This is for instance the objective as defined in GB patent publication No. 2 000 812 where there has been deposited on a first lyaer of TiN, TaN, TaC, ZrN or VN a layer of noble metal or alloy either by evaporation and ionic plating (examples 1 and 2) or by an electrochemical plating followed by a thermal diffusion treatment (examples 3 and 5). It is likewise envisaged to form successively layers of Ti, TiN, a mixture of TiN and noble metal and gold or an alloy of gold. The coatings obtained in all these methods are formed of superposed layers of very different compositions deposited one after the other. Consequently, they run the risk of exhibiting lack of homogeneity or scratch resistance or resistance to thermal shocks to the interfaces between the different layers.
It has been likewise been disclosed in the same British patent publication (example 4) to form following the deposition of a layer of TiN by evaporation and ionic plating, a second layer obtained by the simultaneous evaporation from two distinct sources respectively of Ti and Cu. The preceding comments continue to apply, but furthermore it will be noted that this last procedure does not enable the obtaining of a layer of noble metal on the outer surface of the coating.
In the objective set forth in the above-mentioned document, the superficial gold layer must be thin (equal to or less than 1 .mu.m) in order that the coating produce the expected effect, that is render scarcely noticeable scratches caused by foreign bodies. In this the coating as described does not permit the obtaining of a true plating containing sufficient precious metal so that the article may bear a hallmark.
To overcome the difficulties mentioned in the above cited document, the European patent EP-B No. 0038294 (U.S. Pat. No. 4,403,014) suggests a simple procedure enabling the deposition of hard metallic coatings of which the composition varies progressively from the surface of the article towards the outer surface by means of a source subjected to a single procedure of evaporation or pulverization. Thus, in the course of the deposition, a simultaneous increase is effected in the concentration in volume of precious metal and a decrease in that of a metallic compound (e.g. TiN) these being deposited according to opposing gradients by means of a source including opposite gradients of concentration in volume of precious metal and of the metallic compound. Another method of practising the cited invention starts from separated sources formed respectively by the noble metal and the metallic compound, these sources being subjected to an evaporation procedure in accordance with respectively increasing and decreasing speeds.
The procedure which has just been described exhibits the difficulty of requiring the set up of a composite source, the thickness of which must be related to the thickness of the deposit to be effected, and so requires a delicate adjustment of the deposition apparatus which must be capable of controlling with great precision the variation in speed of the pulverization during the course of the procedure. On the other hand, this procedure leads by definition to deposits of which the volumetric percentage of gold and metallic compound is identical, this not permitting to obtain a deposit of gold of substantial fineness. Finally, it will be noted that this procedure is poorly adapted to the deposition of plating in thick layers since it is slow and burdensome.
The coatings which are objective of the two cited documents, although different in their composition, have a characteristic in common. They both possess a layer of precious metal, for example gold, deposited on underlying layers much harder than that of the precious metal, for example of titanium nitride. If the surface layer is scratched, it will be scratched in a shallow manner and the scratch will be very little apparent, and this less and less so as the underlying layer approaches a colour approximating that of the precious metal deposited at the surface.
The optical, electrical, mechanical and corrosion resistant properties of a film of gold comprising particles of silica (SiO.sub.2) and deposited by a procedure of vapour deposition have been discussed in the review "Thin Solid Films" vol. 39 (1976), p. 165-174, Elsevier Sequoia, Lausanne, Taylor et al: "Properties Of Metal--Dielectric Codeposited Films". The article in question analyses above all the optical properties of such a layer, in particular the spectral reflectivity of pure gold as compared to that of a composite film of gold and of SiO.sub.2. Very succinctly, the article mentions that the scratch resistance of the composite Au-SiO.sub.2 is very good over the entire domain of the composition as examined, said resistance being better than that of a film of pure gold. This comment, purely qualitative, is however not complemented by any quantitative data. The article in question gives no teaching concerning the modifications of optical and mechanical properties due to a non-homogeneous structure (columnar growth at low density, for instance) which might appear for thicknesses of films on the order of 0.4 .mu.m or more.
The Japanese patent document JP-A No. 59 185774 describes plating of a gold alloy deposited in the vapour phase and including furthermore hardening agents present in the form of nitrides. Thanks to this addition, the hardness and corrosion resistance of the plating are improved.
The Japanese patent document JP-A No. 60 114567 proposes the employment of a precious metal codeposited with a transparent oxide in order to form the coating on an article which then will exhibit a better resistance to wear and scratching. There has been proposed the employment of gold together with SiO.sub.2 deposited by methods such as deposition under vacuum. The method may be applied to watch cases and enables reducing the gold content to a fifth of that of a conventional coating, while increasing the resistance to abrasion.
Finally, the Japanese patent document JP-A No. 60 67654 describes a coating formed of gold or of a gold alloy and of titanium nitride deposited onto an article by evaporation under vacuum for instance. Thus resistance to abrasion and scratching may be improved for objects such as watches or frames for glasses.
In the last four documents mentioned above, the emphasis has been placed on the thinness of the deposited layer from whence there results an economy of precious material, the principle concern being to provide a layer having above all wear resistance. No mention has been made of thick layers of precious metal intended to bear a jeweller's hallmark or likewise of thick layers to be coloured while retaining wear resistance.